Method for mechanical stoking in firing installations and firing installation

ABSTRACT

A method for mechanical stoking in firing installations and a firing installation is provided. Fuel is applied to a grate using a mechanical stoker, the grate being disposed in a firing chamber, wherein the mechanical stoker is oriented opposite a delivery direction of the grate. The fuel is introduced directly below a firing chamber ceiling, viewed in the delivery direction of the grate before the end of the grate such that a burnout zone, which does not have fresh fuel applied thereto, is created on the grate between the mechanical stoker and the end of the grate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2009/051623 filed Feb. 12, 2009, and claims the benefit thereof. The International Application claims the benefit of German Application No. 10 2008 010 235.0 DE filed Feb. 21, 2008. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for mechanical stoking in firing plants in which fuel is deposited by means of a mechanical spreader stoker onto a grate which is disposed in the combustion chamber, wherein the direction of throw is oriented opposite to the conveying direction of the grate, as well as to a corresponding firing plant.

BACKGROUND OF INVENTION

The invention can be used in firing plants employing thrower wheel mechanical stoking. In said applications the solid, particulate fuel is introduced into the combustion chamber with the aid of the thrower wheel disposed in the mechanical spreader stoker. The trajectory length can be adjusted by varying the rotational speed of the thrower wheel.

Analogous plants are known for example from DE4217070A1, where coal is broadcast onto a horizontal grate by means of mechanical stoking. According to this arrangement the mechanical spreader stoker is mounted approximately vertically over the end of the grate in the front wall of the combustion chamber.

SUMMARY OF INVENTION

The arrangement of the mechanical spreader stoker over the end of the grate has the disadvantage that fresh fuel also lands on the grate close to the end of the grate and so is no longer able to burn completely before being discharged. This means that it may not be possible to comply with provisions governing the maximum permissible residual carbon content of the ash.

Furthermore, a high rotational speed of the mechanical spreader stoker is necessary to achieve a length of trajectory that reaches as far as the opposite start of the grate.

It is therefore an object of the invention to reduce the residual carbon content of the ash and also to achieve a reduction in the rotational speed of the mechanical spreader stoker.

The object is achieved by means of a method as claimed in the independent claim, according to which the fuel is introduced immediately below the combustion chamber cover, and more specifically, viewed in the direction of conveyance of the grate, ahead of the end of the grate such that there is created on the grate, between the mechanical spreader stoker and the end of the grate, a burnout zone onto which no fresh fuel is deposited.

By virtue of the fact that the mechanical spreader stoker is offset from the end of the grate in the direction of the trajectory it is no longer possible for fresh fuel to reach the last section of the grate before the discharge, but instead only the fuel already in the process of combustion is burned completely. As a result a more complete combustion of the fuel is achieved, thereby bringing about a reduction in the residual carbon content of the ash.

By virtue of the fact that in addition the mechanical spreader stoker is disposed at a higher level a greater length of trajectory can be achieved at the same rotational speed of the thrower wheel or, alternatively, if the length of trajectory remains the same, the rotational speed of the thrower wheel can be reduced.

In this arrangement the thrower wheel is generally disposed horizontally, i.e. the axis of rotation of the thrower wheel is vertical. The fuel is introduced from above in the region of the hub of the thrower wheel into the housing of the mechanical spreader stoker and then conveyed horizontally outward with the aid of the blades of the thrower wheel and delivered—through openings in the mechanical spreader stoker—into the combustion chamber. The axis of rotation of the thrower wheel can, of course, also be inclined to the vertical.

A particularly simple embodiment of the invention provides that the upper part of the front wall of the combustion chamber is offset inwardly into the combustion chamber by means of a lowered part of the combustion chamber cover and the fuel is introduced at the top end of the upper front wall.

The combustion chamber cover closes off the combustion chamber on the top side and has an opening for the purpose of extracting the flue gas.

In order to achieve a particularly low residual carbon content of the ash it can be provided that the burnout zone is equivalent to between 15 and 40%, in particular between 20 and 40%, of the length of the grate.

On account of the almost complete or totally complete combustion on the grate in accordance with the inventive method a post-combustion in an apparatus connected downstream can be dispensed with. Accordingly it can therefore be provided that the combustion residues are extracted from the combustion chamber at the end of the grate.

The method according to the invention can be employed in particular in the case of a forward-acting reciprocating grate. Unlike with a traveling grate, where the grate itself moves in the direction of conveyance, the bars of the grate, for example, remain in position and are simply moved back and forth.

A firing plant corresponding to the method is recited in a further independent claim. Said plant comprises at least one combustion chamber, a grate disposed therein, and a mechanical spreader stoker by means of which fuel can be deposited onto the grate counter to the direction of conveyance of the grate. The firing plant is characterized in that by means of the mechanical spreader stoker the fuel can be introduced immediately below the combustion chamber cover, and more specifically, viewed in the direction of conveyance of the grate, ahead of the end of the grate, with the result that during operation of the mechanical spreader stoker a burnout zone is created on the grate between the mechanical spreader stoker and the end of the grate, onto which burnout zone no fresh fuel can be deposited.

In the case of the firing plant it can be provided in accordance with the inventive method that the burnout zone is equivalent to between 15 and 40%, in particular between 20 and 40%, of the length of the grate.

Similarly, it is provided that at the end of the grate there is a discharge by means of which the combustion residues can be extracted from the combustion chamber, and/or that the grate is a forward-acting reciprocating grate.

A particularly simple embodiment of the firing plant provides that the upper part of the front wall of the combustion chamber is offset inwardly into the combustion chamber by means of a lowered part of the combustion chamber cover and an opening for introducing the fuel is provided at the top end of the upper front wall.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is shown in the figure and explained below.

DETAILED DESCRIPTION OF INVENTION

The figure schematically shows the combustion chamber 1 of a firing plant in which a grate 2 which conveys from left to right is disposed horizontally. Said forward-acting reciprocating grate consists of individual elements having bars oriented in the direction of conveyance (in this example the bars have a length of 500 mm), with only every second element being moved back and forth. The fuel 3 is supplied from above to a mechanical spreader stoker 4 and conveyed by means of the latter into the combustion chamber 1.

The mechanical spreader stoker 4 has a horizontally disposed thrower wheel, i.e. the axis of rotation of the thrower wheel is vertical. The fuel 3 is introduced into the mechanical spreader stoker 4 from above in the region of the hub of the thrower wheel and then conveyed horizontally outward with the aid of the blades of the thrower wheel and delivered—through openings in the mechanical spreader stoker—into the combustion chamber. As already mentioned previously, the axis of rotation of the thrower wheel can also be inclined to the vertical.

In this arrangement the mechanical spreader stoker 4 itself is disposed outside of the combustion chamber 1, with the openings of the mechanical spreader stoker 4 leading into an opening 6 of the combustion chamber 1, and more specifically in such a way that the fuel can be discharged as closely as possible below the combustion chamber cover 5. The fall curves of the fuel conveyed into the combustion chamber are represented by curves 7. Toward that end, from the burnout-zone-side end of the combustion chamber 1 as far as the opening 6 for the fuel, the combustion chamber cover 5 is embodied at a lower level, with the result that there is space for the mechanical spreader stoker 4 above said lowered part 5 a of the combustion chamber cover 5. The opening 6 of the combustion chamber is situated in the upper front wall 8 that is to be formed as a result of the lowering. The lower front wall 13 of the combustion chamber 1 remains unchanged. In the present example the front wall 8, 13 of the combustion chamber 1 is disposed vertically in this case. The front wall 8, 13 could, however, also be completely or partly disposed in an inclined attitude, in which case the vertical component should be preponderant. As soon as the horizontal component is preponderant, which is to say that the wall is inclined by more than 45° to the vertical, said wall must already be associated with the combustion chamber cover 5.

The combustion chamber cover 5 closes off the combustion chamber 1 on the top side and has an opening for the purpose of extracting the flue gas 9. In this example a substantial part of the combustion chamber cover is disposed horizontally, said part in this case comprising about half the length of the grate 5. Of course, the combustion chamber cover can also have any other shape, this being dependent on the particular application, in particular on the fuel. In other applications the opening for extracting the flue gas 9, for example, can be larger or smaller.

By virtue of the fact that the opening 6 for the fuel from the mechanical spreader stoker 4 is displaced inwardly into the combustion chamber counter to the direction of conveyance of the grate 5 there is created on the grate 5 the burnout zone 10, into which no fresh fuel 7 is able to make its way directly from the mechanical spreader stoker 4. The burnout zone 10 therefore extends at least from vertically below the opening 6, i.e. the upper front wall 8, as far as the discharge-side end of the grate 5. In this embodiment the burnout zone is equal to approximately one sixth of the length of the grate.

The fresh fuel 7 from the mechanical spreader stoker 4 can only land in the remaining section 11 of the grate 5. Combustion therefore commences in this section 11 and continues in the burnout zone 10 until finally the ash 12 remaining at the end of the grate 5 is conveyed without post-combustion from the combustion chamber 1.

A traveling grate can alternatively be used instead of a forward-acting reciprocating grate. Moreover, the grate does not have to be horizontal, but can also be disposed at a slightly inclined attitude, e.g. by up to 7° to the horizontal. In addition to the embodiment shown here by way of example the grate can also be implemented in stages. 

1.-8. (canceled)
 9. A method for mechanical stoking in firing plants including a combustion chamber with a combustion chamber cover, comprising: depositing fuel onto a grate of the combustion chamber by a mechanical spreader stoker; orienting a direction of throw opposite to the direction of conveyance of the gate, wherein the fuel is introduced immediately below the combustion chamber cover, and, viewed in the direction of conveyance of the grate, ahead of an end of the grate, such that a burnout zone is created between the mechanical spreader stoker and the end of the grate, wherein no fuel is deposited in the burnout zone; offsetting an upper part of a front wall of the combustion chamber inwardly into the combustion chamber by a lowered part of the combustion chamber cover, wherein the fuel is introduced at a top end of an upper front wall; and providing the mechanical spreader stoker with a thrower wheel including an at least approximately vertical axis of rotation.
 10. The method as claimed in claim 9, wherein a length of the burnout zone is between 15% and 40% of a length of the grate.
 11. The method as claimed in claim 9, wherein a length of the burnout zone is between 20% and 40% of a length of the grate.
 12. The method as claimed in claim 9, further comprising: extracting combustion residues from the combustion chamber at the end of the grate.
 13. The method as claimed in claim 9, wherein the grate is a forward-acting reciprocating grate.
 14. A firing plant, comprising: a combustion chamber; a combustion chamber cover; a grate disposed in the combustion chamber; a mechanical spreader stoker, wherein fuel is deposited onto the grate opposite to a direction of conveyance of the grate, the fuel is introduced by the mechanical spreader stoker immediately below the combustion chamber cover, and, viewed in the direction of conveyance of the grate, ahead of the end of the grate such that during operation of the mechanical spreader stoker a burnout zone is created between the mechanical spreader stoker and an end of the grate, no fuel being deposited in the burnout zone, an upper part of a front wall of the combustion chamber is offset inwardly into the combustion chamber by a lowered part of the combustion chamber cover, an opening is provided at a top end of an upper front wall for introducing the fuel, and the mechanical spreader stoker comprises a thrower wheel with an at least approximately vertical axis of rotation.
 15. The firing plant as claimed in claim 14, wherein a length of the burnout zone is between 15% and 40% of a length of the grate.
 16. The firing plant as claimed in claim 14, wherein a length of the burnout zone is between 20% and 40% of a length of the grate.
 17. The firing plant as claimed in claim 14, further comprising: a discharge at the end of the grate for extracting combustion residues from the combustion chamber.
 18. The firing plant as claimed in claim 14, wherein the grate is a forward-acting reciprocating grate. 